Memory cell constructions

ABSTRACT

Some embodiments include methods for fabricating memory cell constructions. A memory cell may be formed to have a programmable material directly against a material having a different coefficient of expansion than the programmable material. A retaining shell may be formed adjacent the programmable material. The memory cell may be thermally processed to increase a temperature of the memory cell to at least about 300° C., causing thermally-induced stress within the memory cell. The retaining shell may provide a stress which substantially balances the thermally-induced stress. Some embodiments include memory cell constructions. The constructions may include programmable material directly against silicon nitride that has an internal stress of less than or equal to about 200 megapascals. The constructions may also include a retaining shell silicon nitride that has an internal stress of at least about 500 megapascals.

TECHNICAL FIELD

Memory cell constructions, and methods for fabricating memory cellconstructions.

BACKGROUND

Memory is one type of integrated circuitry, and is used in computersystems for storing data. Integrated memory is usually fabricated in oneor more arrays of individual memory cells. The memory cells areconfigured to retain or store memory in at least two differentselectable states. In a binary system, the states are considered aseither a “0” or a “1”. In other systems, at least some individual memorycells may be configured to store more than two levels or states ofinformation.

Integrated circuit fabrication continues to strive to produce smallerand denser integrated circuits. There is a continuing effort to reducethe number of components in individual devices because such can reducethe size of finished constructions, and can simplify processing. Thesmallest and simplest memory cell will likely be comprised of twoelectrodes having a programmable material received between them.

Suitable programmable materials have two or more selectable memorystates to enable storing of information by an individual memory cell.The reading of the cell comprises determination of which of the statesthe programmable material is in, and the writing of information to thecell comprises placing the programmable material in a predeterminedstate.

Phase change materials, such as ovonic memory materials (for instance,various chalcogenides), are being considered for utilization asprogrammable materials in memory cells. The phase change materialstransform from one phase to another through application of appropriateelectrical stimulus, with each phase corresponding to a different memorystate. The ovonic memory materials may be utilized in combination withselection devices, such as diodes, transistors, or ovonic thresholdswitches.

A problem encountered in the utilization of ovonic materials isdelamination of the ovonic materials from adjacent materials. Suchdelamination can be detrimental to memory cell performance, and in somecases may lead to failure of memory cells. It would be desirable todevelop improvements which alleviate or prevent the delaminationproblems encountered during utilization of ovonic materials.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic, cross-sectional view of a portion of asemiconductor construction, and illustrates an example embodiment memorycell construction.

FIG. 2 is a diagrammatic, cross-sectional view of a portion of anothersemiconductor construction, and illustrates another example embodimentmemory cell construction.

FIGS. 3-5 are diagrammatic, cross-sectional views of a portion of asemiconductor construction at various process stages of an exampleembodiment method of forming a memory cell construction.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Some embodiments include methods of offsetting thermally-inducedstresses in memory cells. Such methods may include provision ofretaining material shells that partially surround the memory cells, withsuch shells being configured to impart stresses that substantiallybalance thermally-induced stresses. In some embodiments, delaminationand other thermally-induced problems associated with phase change randomaccess memory (PCRAM) may be alleviated or prevented through utilizationof retaining material shells.

Example embodiments are described with reference to FIGS. 1-5.

Referring to FIG. 1, a semiconductor construction 10 includes a memorycell 14 supported over a substrate 12.

The memory cell includes a bottom electrode 18, a programmable material20 over the bottom electrode, and a top electrode 22 over theprogrammable material.

The substrate 12 may comprise any suitable material or combination ofmaterials, and in some embodiments may comprise one or more electricallyinsulative materials supported over a semiconductor material. Althoughsubstrate 12 is shown to be homogenous, the substrate may comprisenumerous materials. For instance, in some embodiments the substrate 12may comprise silicon dioxide over monocrystalline silicon. Suchsubstrate may be referred to as a semiconductor substrate, or as aportion of a semiconductor substrate; with the terms “semiconductivesubstrate,” “semiconductor construction” and “semiconductor substrate”meaning any construction comprising semiconductive material, including,but not limited to, bulk semiconductive materials such as asemiconductive wafer (either alone or in assemblies comprising othermaterials), and semiconductive material layers (either alone or inassemblies comprising other materials). The term “substrate” refers toany supporting structure, including, but not limited to, thesemiconductive substrates described above.

An electrically insulative material 16 is over substrate 12, and thebottom electrode 18 of memory cell 14 extends through the electricallyinsulative material 16. In some embodiments, the insulative material 16and electrode 18 may be considered to form a base which supports theprogrammable material 20. In the shown embodiment, a bottom surface ofthe programmable material directly contacts both the bottom electrode 18and the insulative material 16 of such base.

In some embodiments, the electrically insulative material 16 maycomprise, consist essentially of, or consist of low-stress siliconnitride; with “low-stress” silicon nitride being silicon nitride havingan internal stress of less than or equal to 200 megapascals. Thelow-stress silicon nitride may be formed with any suitable processing.Example processing which may be utilized to form silicon nitride havingan internal stress of less than or equal to 200 megapascals can includeplasma-enhanced chemical vapor deposition (PECVD), atomic layerdeposition (ALD) and/or low-pressure chemical vapor deposition (LPCVD).

The bottom electrode 18 comprises a bottom electrode material 24. Suchmaterial may comprise any suitable composition or combination ofcompositions; and in some embodiments may comprise, consist essentiallyof, or consist of one or more of TiSiN, TiN, and TaSiN (where theformulas indicate the components within the listed compounds, ratherthan designating specific stoichiometries of such components).

The programmable material 20 may comprise any suitable composition orcombination of compositions; and in some embodiments may comprise ovonicmaterial, such as chalcogenide or other suitable phase change material.For instance, in some embodiments the programmable material 20 maycomprise, consist essentially of, or consist of a composition containinggermanium, antimony and tellurium; such as, for example, Ge₂Sb₂Te₅.

The top electrode 24 is shown to comprise two different electricallyconductive materials 26 and 28, with the material 26 being directlyagainst the programmable material 20. In some embodiments, the material26 may comprise, consist essentially of, or consist of TiN or WN (wherethe formulas indicate the components within the listed compounds, ratherthan designating specific stoichiometries of such components); and thematerial 28 may comprise, consist essentially of, or consist oftungsten. In some embodiments, the memory cell may have a width within arange of from about 20 nanometers to about 200 nanometers across the topelectrode along the cross-section of FIG. 1.

The memory cell 14 may be one of a large plurality of identical memorycells incorporated into a memory array. The bottom and top electrodes 18and 22 may be comprised by, or electrically coupled to, access/senselines (i.e., wordlines/bitlines). The individual memory cells of thememory array may be uniquely addressed through combinations ofaccess/sense lines to enable the individual memory cells to beprogrammed and read during various operations of the memory array. Insome embodiments, the top electrode 22 may be configured as anaccess/sense line extending in and out of the page relative to the viewof FIG. 1, and the bottom electrode 18 may be coupled to an access/senseline (not shown) that extends substantially perpendicularly to the topelectrode line.

Select devices (not shown), such as diodes, transistors, or ovonicthreshold switches (OTSs), may be coupled with the memory cells of thePCRAM array to alleviate undesired leakage within the memory array. Aselect device may be between an access/sense line and the programmablematerial of a memory cell in some embodiments, and may be on an opposingside of an access/sense line from the programmable material in otherembodiments.

An electrically insulative material 38 is over the memory cell 14, andmay be utilized for electrical isolation of the illustrated memory cellfrom adjacent memory cells (not shown) of a memory array. Theelectrically insulative material 38 may comprise any suitablecomposition or combination of compositions; and in some embodiments maycomprise, consist essentially of, or consist of one or more of silicondioxide and any of various doped silicon oxides. Example doped siliconoxides are borophosphosilicate glass (BPSG), phosphosilicate glass(PSG), fluorosilicate glass (FSG), etc.

As discussed above in the “Background” section of this disclosure, aproblem encountered with some memory cells is delamination of ovonicmaterial from other materials. An aspect of some of the embodimentsprovided herein is recognition that the delamination may be due, atleast in part, to thermal mismatch between various materials of thememory cells. For instance, the memory cell of FIG. 1 may have asubstantially different coefficient of expansion of the programmablematerial 20 relative to one or more of the electrically insulativematerial 16, the electrically insulative material 38, the bottomelectrode material 24, the top electrode material 26 and the topelectrode material 28. For example, Ge₂Sb₂Te₅ may have a thermalexpansion coefficient of 18.1×10⁻⁶/C.°, SiO₂ may have a thermalexpansion coefficient of 0.57×10⁻⁶/C.°, BPSG may have a thermalexpansion coefficient of 0.6×10⁻⁶/C.°, low-stress silicon nitride mayhave a thermal expansion coefficient of 3.0×10⁻⁶/C.°, TiN may have athermal expansion coefficient of 12×10⁻⁶/C.°, and W may have a thermalexpansion coefficient of 4.5×10⁻⁶/C.°.

The different rates of thermal expansion of the various materials of thememory cell can create mechanical stresses within the memory cell thatmay ultimately lead to delamination of the programmable material fromone or more of the materials directly against such programmablematerial. Persons of ordinary skill in the art will recognize thatmechanical stresses may be considered to be a sum of internal stressesand external stresses. For instance, the individual materials of amemory cell may have internal stresses associated with, among otherthings, the compositions of the materials, the lattice configurations ofthe materials, etc.; and may have external stresses induced by theenvironments surrounding such materials.

An aspect of some embodiments is recognition that it may be possible tocompensate for the thermally-induced stresses of the memory cell byproviding an internally-stressed retaining shell at least partiallyaround the programmable material of the memory cell. Such retainingshell may substantially balance the thermally-induced stresses, andthereby alleviate or prevent thermally-induced delamination fromoccurring within the memory cell. The thermally-induced stresses may bereferred to as first stresses, and the stress provided by the retainingshell may be referred to as a second stress. In some applications, thesecond stress may be at least about equal to the first stresses underpre-defined thermal conditions to substantially balance the firststresses when the memory cell is exposed to such conditions.

A memory cell may be at a temperature below about 100° C. (for instance,the memory cell may be at a room temperature of about 25° C.)as-fabricated, and then may be heated to a higher temperature duringfabrication of various components and materials that are subsequentlyformed as part of the integrated circuit comprising the memory cell.Thermally-induced delamination may become problematic during the thermaltransition of a memory cell to processing temperatures which equal orexceed about 300° C. (i.e., during a thermal transition of the memorycell across a temperature differential of at least about 200° C.).Accordingly, some aspects include utilization of a retaining shellconfigured to balance thermally-induced stresses that manifest duringthermal processing to temperatures of at least about 300° C. Inparticular applications, the retaining shell may be configured tobalance thermally-induced stresses that manifest during thermalprocessing to temperatures of at least about 400° C. (i.e., during athermal transition of the memory cell which increases a temperature ofthe cell by at least about 300° C.), and in some applications theretaining shell may be configured to balance thermally-induced stressesthat manifest during thermal processing to temperatures of at leastabout 450° C. (i.e., during a thermal transition of the memory cellwhich increases a temperature of the cell by at least about 350° C.).

In the embodiment of FIG. 1, a retaining shell 32 extends partiallyaround a periphery of memory cell 14, with such retaining shellcomprising a retaining shell material 30. The retaining shell material30 may comprise any suitable composition or combination of compositions,and in some embodiments may comprise one or both of an electricallyinsulative material and an electrically conductive material. Forinstance, in some embodiments the retaining shell material may comprise,consist essentially of or consist of one or more of metal, carbon,silicon nitride and silicon dioxide. If the retaining shell materialcomprises metal, such metal may be in elemental form (for instance,elemental copper, etc.), may be an alloy or mixture, or may be ametal-containing compound (metal nitride, metal silicide, etc.).

In some embodiments, the retaining shell material 30 may comprisehigh-stress silicon nitride; with “high-stress” silicon nitride beingsilicon nitride having an internal stress of at least about 500megapascals. Such high-stress second nitride may be formed utilizing anysuitable methodology. Example methodology for forming the high-stresssilicon nitride may include one or more of PECVD, LPCVD and ALD,utilizing tailored conditions which enhance hydrogen removal duringgrowth of the silicon nitride. For instance, high stress silicon nitridemay be formed utilizing a multi-layer deposition technique with plasmaas an energy source. The plasma can provide energy to break Si—H and N—Hbonds in deposited films (i.e., layers), leading to enhanced hydrogenremoval. As another example, ultraviolet assisted thermal processing(UVTP) may be utilized in a deposition process, with UV photons being anenergy source. The UV photon energy can break bonds, and hydrogen fromneighboring broken bonds can then combine to form molecular H₂. Themolecular H₂ can diffuse out, leading to enhanced hydrogen removal.

The retaining shell material 30 may be formed to any suitable thickness.In embodiments in which the retaining shell material consists ofhigh-stress silicon nitride, the retaining shell material may be formedto a thickness of, for example, about 20 nanometers.

In the shown embodiment, the retaining shell material 30 is directlyagainst the insulative material 16. Thus, in embodiments in whichretaining shell material 30 comprises high-stress silicon nitride andinsulative material 16 comprises low-stress silicon nitride, thehigh-stress silicon nitride of the retaining shell material may bedirectly against low-stress silicon nitride of the insulative material.

The programmable material 20 has a periphery 40 along the cross-sectionof the view of FIG. 1, with such periphery including a bottom surface41, sidewall surfaces 43, and a top surface 45. The retaining shell 32is shown to surround about three-fourths of such periphery in theembodiment of FIG. 1. In other embodiments, the retaining shell may havea different configuration in order to provide a desired stress balance.In some example embodiments, the retaining shell may surround at leastabout one-half of the periphery of the programmable material along across-section (for instance, the cross-section of the view of FIG. 1),the retaining shell may surround at least about two-thirds of suchperiphery of the programmable material, etc.

The stress provided by the retaining shell may have oppositecharacteristics to thermally-induced stresses within the memory cell insome embodiments. For instance, the thermally-induced stresses may beprimarily compressive stresses, and the stress provided by the retainingshell may be primarily tensile; or vice versa.

The retaining shell 32 is illustrated in FIG. 1 as a single homogeneousmaterial. In some embodiments, the retaining shell may be heterogeneous.In some embodiments, the heterogeneous retaining shell may comprise alaminate of two or more different materials. FIG. 2 illustrates asemiconductor construction 10 a analogous to the construction 10 of FIG.1, but having a retaining shell 32 a which is a laminate of twodifferent retaining shell materials 50 and 52. The materials 50 and 52may be considered to be an inner retaining shell material and an outerretaining shell material, respectively. The inner and outer retainingshell materials are shown to be different thickness relative to oneanother. In other embodiments, the inner and outer retaining shellmaterials may have other relative thicknesses than is shown in FIG. 2,and in some embodiments the inner and outer retaining show materials maybe about the same thickness as one another. Although the illustratedlaminate has two different materials, in other embodiments the laminatemay have three or more materials. If the laminate has three or morematerials, all of the materials may be different in composition relativeto one another, or two or more materials may have a same composition asone another.

In the shown embodiment, both of the inner and outer retaining shellmaterials 50 and 52 extend along the sidewall surfaces of programmablematerial 20, as well as along an upper surface of insulative material16, and along the upper surface of top electrode 22. In otherembodiments, one of the materials 50 and 52 may be only along thesidewall surfaces 43 of the programmable material, while the other ofthe materials 50 and 52 extends further around the periphery 40 of theprogrammable material. For instance, in some embodiments the innerretaining shell material 50 may be a liner which protects sidewallsurfaces 43 of the programmable material from reaction with the outerretaining shell material.

In some example embodiments, the inner retaining shell material 50 maycomprise silicon dioxide, while the outer retaining shell material 52comprises high-stress silicon nitride. In some example embodiments, oneof the retaining shell materials 50 and 52 may comprise silicon nitridewhile the other comprises carbon. In some example embodiments, one ofthe materials 50 and 52 may comprise carbon while the other comprisessilicon carbide.

In the shown embodiment, the materials 50 and 52 are referred to as bothbeing retaining shell materials, which implies that both of thematerials 50 and 52 provide stresses which are utilized to offsetthermally-induced stresses of the memory cell. In some embodiments,constructions similar to 10 a may be formed, but only one of thematerials 50 and 50 a will provide stresses utilized to offsetthermally-induced stresses within the memory cell. For instance, in someembodiments, a material analogous to the material 52 may be a retainingshell material while a material analogous to the material 50 onlyfunctions as a spacer which displaces the retaining shell material fromone or more of the underlying materials 16, 20, 26 and 28.

In some embodiments, one or more materials of a memory cell may betailored to assist the retaining shell in canceling or alleviatingthermally-induced stresses. For instance, one or both of theelectrically conductive materials 26 and 28 may be tailored to assist incanceling or alleviating thermally-induced stresses.

The retaining shells discussed above may be incorporated into memorycell constructions utilizing any suitable processing. For instance,FIGS. 3-5 illustrate example processing that may be utilized to form theconstruction 10 of FIG. 1.

Referring to FIG. 3, construction 10 is shown at a processing stageprior to that of FIG. 1. The construction has the insulative material 16over substrate 12, and has the bottom electrode 18 extending through theinsulative material 16. Also, the construction has the programmablematerial 20 and the top electrode 22 formed over the bottom electrode18. The programmable material 20 and the top electrode 22 are patternedinto a memory cell configuration with sidewalls 62 extending upwardlyfrom an upper surface of insulative material 16. The sidewall surfaces62 include sidewall surfaces 27 of conductive material 26, sidewallsurfaces 29 of conductive material 28, and sidewall surfaces 43 ofprogrammable material 20. In some embodiments, the sidewall surfaces 43,27 and 29 may be considered to be coextensive with one another along thecross-section shown in FIG. 3.

Referring to FIG. 4, retaining shell material 30 is formed overinsulative material 16, along the sidewalls 62 of the memory cellconfiguration, and across an upper surface of the top electrode 22. Theretaining shell material may be formed utilizing any suitableprocessing, including, for example, one or more of the processesdiscussed above with reference to FIG. 1. In some example embodiments,the retaining shell material 30 and the insulative material 16 may bothcomprise silicon nitride, and thus may comprise a common composition asone another. However, the silicon nitride of the retaining shellmaterial may be high-stress silicon nitride, while the silicon nitrideof insulative material 16 may be low-stress silicon nitride.

Referring to FIG. 5, the electrically insulative material 38 is formedover retaining shell material 30 to form the construction 10 describedabove with reference to FIG. 1. In subsequent processing, suchconstruction may be thermally processed to a temperature in excess of300° C. Such thermal processing may be utilized for fabrication of othercomponents (not shown) of integrated circuitry supported by substrate12. The thermal processing may induce a first stress due to thermalmismatch of the programmable material 20 and other materials directlyadjacent to such programmable material, and the retaining shell material30 may be tailored in composition and configuration to provide a secondstress which is at least about equal to the first stress and which isconfigured to substantially balance the first stress. The retainingshell material can thus be configured to avoid delamination of theprogrammable material and/or to avoid other problems that may otherwiseresult from the thermally-induced first stress. Additionally, in someembodiments, one or more materials of the memory cell may be tailored toassist the retaining shell in canceling or alleviating thermally-inducedstresses. For instance, one or both of the electrically conductivematerials 26 and 28 may be tailored to assist in canceling oralleviating thermally-induced stresses.

Processing analogous to that of FIGS. 3-5 may be utilized to form aconstruction in which the retaining shell material comprises a laminateof two or more different materials, such as, for example, a constructionanalogous to that of FIG. 2.

The electronic devices discussed above may be incorporated intoelectronic systems. Such electronic systems may be used in, for example,memory modules, device drivers, power modules, communication modems,processor modules, and application-specific modules, and may includemultilayer, multichip modules. The electronic systems may be any of abroad range of systems, such as, for example, clocks, televisions, cellphones, personal computers, automobiles, industrial control systems,aircraft, etc.

The particular orientation of the various embodiments in the drawings isfor illustrative purposes only, and the embodiments may be rotatedrelative to the shown orientations in some applications. The descriptionprovided herein, and the claims that follow, pertain to any structuresthat have the described relationships between various features,regardless of whether the structures are in the particular orientationof the drawings, or are rotated relative to such orientation.

The cross-sectional views of the accompanying illustrations only showfeatures within the planes of the cross-sections, and do not showmaterials behind the planes of the cross-sections in order to simplifythe drawings.

When a structure is referred to above as being “on” or “against” anotherstructure, it can be directly on the other structure or interveningstructures may also be present. In contrast, when a structure isreferred to as being “directly on” or “directly against” anotherstructure, there are no intervening structures present. When a structureis referred to as being “connected” or “coupled” to another structure,it can be directly connected or coupled to the other structure, orintervening structures may be present. In contrast, when a structure isreferred to as being “directly connected” or “directly coupled” toanother structure, there are no intervening structures present.

In compliance with the statute, the subject matter disclosed herein hasbeen described in language more or less specific as to structural andmethodical features. It is to be understood, however, that the claimsare not limited to the specific features shown and described, since themeans herein disclosed comprise example embodiments. The claims are thusto be afforded full scope as literally worded, and to be appropriatelyinterpreted in accordance with the doctrine of equivalents.

1. A memory cell construction, comprising: a bottom electrode; aprogrammable material over the bottom electrode; a top electrode overthe programmable material; and a retaining shell that extends at leastabout half-way around an outer periphery of the programmable materialalong a cross-section through the programmable material and theretaining shell; the retaining shell extending along opposing sidewallsurfaces of the programmable material, and extending over the topelectrode; the retaining shell comprising material having an internalstress of at least about 500 megapascals.
 2. The memory cellconstruction of claim 1 wherein the retaining shell material iselectrically insulative.
 3. The memory cell construction of claim 1wherein the retaining shell material is electrically conductive.
 4. Thememory cell construction of claim 1 wherein the retaining shell materialcomprises a metal.
 5. The memory cell construction of claim 1 whereinthe retaining shell material comprises carbon.
 6. The memory cellconstruction of claim 1 wherein the retaining shell material comprisessilicon nitride.
 7. The memory cell construction of claim 1 wherein theprogrammable material comprises germanium, antimony and tellurium.
 8. Amemory cell construction, comprising: a bottom electrode; a programmablematerial over the bottom electrode; a top electrode over theprogrammable material; a retaining shell that extends at least abouthalf-way around an outer periphery of the programmable material along across-section through the programmable material and the retaining shell;the retaining shell extending along opposing sidewall surfaces of theprogrammable material, and extending over the top electrode; theretaining shell comprising material having an internal stress of atleast about 500 megapascals; and wherein the retaining shell material isa first retaining shell material, and wherein the retaining shellcomprises a laminate of the first retaining shell material and a secondretaining shell material.
 9. The memory cell construction of claim 8wherein one of the first and second retaining shell materials comprisesone or more of metal, carbon, silicon carbide and silicon nitride. 10.The memory cell construction of claim 8 wherein one of the first andsecond retaining shell materials comprises silicon carbide, and theother of the first and second retaining shell materials comprisescarbon.
 11. The memory cell construction of claim 8 wherein one of thefirst and second retaining shell materials comprises silicon nitride.12. A memory cell construction, comprising: a bottom electrode; aprogrammable material over the bottom electrode; a top electrode overthe programmable material; a retaining shell that extends at least abouthalf-way around an outer periphery of the programmable material along across-section through the programmable material and the retaining shell;the retaining shell extending along opposing sidewall surfaces of theprogrammable material, and extending over the top electrode; theretaining shell comprising material having an internal stress of atleast about 500 megapascals; and silicon dioxide directly between theretaining shell material and the sidewall surfaces of the programmablematerial.
 13. A memory cell construction, comprising: a programmablematerial directly against a first silicon nitride; said first siliconnitride having an internal stress of less than or equal to about 200megapascals; and a retaining shell that extends at least about half-wayaround an outer periphery of the programmable material along across-section through the retaining shell and the programmable material;the retaining shell comprising a second silicon nitride having aninternal stress of at least about 500 megapascals.
 14. The memory cellconstruction of claim 13 wherein a region of the second silicon nitrideis directly against a region of the first silicon nitride.
 15. Thememory cell construction of claim 13 wherein the programmable materialcomprises germanium, antimony and tellurium.
 16. The memory cellconstruction of claim 15 further comprising a bottom electrode under abottom surface of the programmable material and a top electrode directlyover a top surface of the programmable material; the bottom electrodeextending through the first silicon nitride; wherein the retaining shellextends along opposing sidewall surfaces of the programmable materialalong the cross-section, and extends over the top electrode.
 17. Thememory cell construction of claim 16 further comprising silicon dioxidedirectly between the retaining shell and the sidewall surfaces of theprogrammable material.